The location and regulation of the transcapillary pathway/s for large molecules (greater than 1.5 nm radius) remains unresolved. Whole organ studies suggest greater than 50% of macromolecular flux occurs across a large pore pathway; it is not known if the pathway/s exists in all exchange vessels or is confined to one portion of the vasculature. The first aim is to test the hypothesis that the primary site of large molecule exchange from plasma to tissue is in the venous microvasculature. It has not been resolved if the pathway/s are the same in different tissues nor is it clear how the permeability of the microvasculature changes to meet the metabolic demands of the tissue. The second aim is to resolve if the mechanisms determining large molecule transport differ from a metabolically quiescent and a metabolically active tissue. To address these problems, single, perfused microvessel methods will be used to measure water and solute flux under strictly defined physicochemical conditions on vessels from the mesentery and skeletal muscle of the frog. The novel approach to these classical problems is the ability to (a) partition macromolecular flux to distinguish the diffusive flux and the flux coupled to water flow (solvent drag), (b) define the anatomical location of microvessels of known solute and water permeability, and (c) define the large molecule pathway/s across identified microvessels by measuring the permeability to solutes of known physicochemical properties. These studies are of importance because both short and long-term fluid balance depends on the distribution of macromolecules between circulating blood and tissue. Vessel wall integrity is compromised under clinically relevant conditions of trauma, edema, and inflammation. In these states, large molecules and water, normally retained in the vasculature leak into the surrounding tissue, edema ensues, and organ function may be lost. Clarification of the nature and location of large solute pathways will provide the basic information needed to initiate clinical intervention of conditions such as pulmonary edema, venous congestion, and shock.